Not Applicable
Not Applicable
1. Field of Invention
This invention pertains to the removal of siloxanes from a waste gas stream containing siloxanes and H2O.
More particularly, this invention pertains to a system for sequentially removing first H2O in a primary heat exchanger and then H2O and siloxanes in alternating gas-refrigerant heat exchangers to provide continuous removal of siloxanes from a waste gas stream.
A secondary benefit of the invention is the significant reduction in the amount of numerous other contaminants e.g. hydrogen sulfide, sulfur dioxide, halogens, volatile organic compounds (VOC), etc., commonly present in the waste gas stream. Washing of the gas and solubility of the contaminants in the condensed moisture, as well as the attraction between neighboring atoms by van der waals forces cleanse the gas even more and convert it into a useful xe2x80x9cgreen energy source.xe2x80x9d
2. Description of the Related Art
Landfills and digesters generate substantial amounts of waste gas streams containing methane. It is desirable to use this methane as fuel for boilers, turbines and other energy sources, particularly in contrast to allowing it to escape into the atmosphere, where it exacerbates the xe2x80x9cgreenhouse effect.xe2x80x9d Unfortunately, the waste gas streams collected from landfills and digesters also contain various other organic compounds, some of which are quite damaging to the boilers, combustion engines, turbines and the systems used to treat the exhaust gases generated upon burning the waste gas.
One family of compounds that has proven to be particularly troublesome when burning waste gases is siloxanes, cyclic organic silicon monomers. Siloxanes are widely used as dispersion agents in various consumer products, including deodorants, shampoos and shaving cream. In addition, siloxanes are used in a variety of industrial applications and are periodically discharged in wastewater. Accordingly, it is quite common for siloxanes to be found in landfills and wastewater.
Siloxanes are frequently volatile, having a dew point of about xe2x88x929xc2x0 F., and therefore the waste gas streams from landfills and digesters generally contain siloxanes. When the waste gas is burned, the silicon contained in the siloxanes is deposited on the turbine and engine parts or boiler tubes, for example, reducing the efficiency of the energy generating equipment. In addition, the selective catalytic reduction equipment used to remove NOx is particularly sensitive to fouling by silicon.
Various efforts have been made to remove siloxanes from the waste gas streams prior to burning. For example, activated carbon filters have been used, but the activated carbon must be regenerated periodically in a kiln. Filtering resins and collection in methanol and tetraglyme have also been used. Costs have been prohibitive and regeneration of the resins has proven to be quite difficult.
It has been recognized that cooling a waste gas stream to a temperature of xe2x88x9210xc2x0 to xe2x88x9220xc2x0 F. results in substantially complete removal of siloxanes from a waste gas stream. Ed Wheless and Dan Gary, Siloxanes in Landfill And Digester Gas, 25th Annual Landfill Symposium, Solid Waste Association of North America, 2002. However, chilling the raw waste gas below the freezing temperature of water rapidly clogs the heat exchanger tubes with frozen condensate.
It is an object of the present invention to provide a cost effective system for removing H2O, siloxanes and other substances soluble in the condensate from waste gas streams.
It is also an object of the present invention to provide a system for continuously removing H2O, siloxanes and other substances soluble in the condensate from waste gas streams.
According to one embodiment of the present invention, a waste gas stream, which may have a temperature as high as 300xc2x0 F., is directed to a primary gas-to-gas heat exchanger, whereby the waste gas is chilled to a temperature close to, but above, 32xc2x0 F., to condense a substantial portion of the H2O carried in the waste gas stream. The condensing H2O also collects a portion of other impurities in the waste gas, including siloxanes. The cooled waste gas is then directed to a first of two gas-refrigerant heat exchangers, whereby the temperature of the waste gas is reduced to about xe2x88x9220xc2x0 F. Within the first gas-refrigerant heat exchanger, the remaining H2O and the siloxanes are condensed and removed. Over time, the frozen H2O begins to block the passage of waste gas through the first gas-refrigerant heat exchanger. Before a substantial blockage occurs, the chilled waste gas stream is diverted to a second gas-refrigerant heat exchanger operating in substantially the same manner as the first gas-refrigerant heat exchanger. Simultaneously, the first gas-refrigerant heat exchanger is defrosted using a defrosting fluid, e.g. a refrigerant, to remove the frozen H2O and collected siloxanes. The first and second gas-refrigerant heat exchangers alternate between freezing and defrosting cycles to provide continuous removal of H2O and siloxanes from the waste gas stream.
The cleansed and dry waste gas stream alternatingly exits either of the first or second gas-refrigerant heat exchanger at a temperature of about xe2x88x9220xc2x0 F. and is used as the coolant gas for the primary gas-gas heat exchanger that provides initial cooling of the waste gas, prior to discharge to points of use.